supplementary materials

The title compound, Sr2Mn3(HPO4)2(PO4)2, was synthesized under hydrothermal conditions. In the structure, one of two Mn atoms is located on an inversion centre, whereas all others atoms are located in general positions. The framework structure is built up from two types of MnO6 octahedra (one almost undistorted, one considerably distorted), one PO3OH and one PO4 tetrahedron. The centrosymmetric MnO6 octahedron is linked to two other MnO6 octahedra by edge-sharing, forming infinite zigzag chains parallel to [010]. The PO3OH and PO4 tetrahedra connect these chains through common vertices or edges, resulting in the formation of sheets parallel to (100). The Sr2+ cation is located in the interlayer space and is bonded to nine O atoms in form of a distorted polyhedron and enhances the cohesion of the layers. Additional stabilization is achieved by a strong interlayer O-HO hydrogen bond between the PO3OH and PO4 units. The structure of the title phosphate is isotypic to that of Pb2Mn3(HPO4)2(PO4)2.

Widespread studies were devoted to metal-based phosphates, either with
open-framework structures, or in terms of porous materials. Within those
materials, the anionic framework, generally constructed from PO4
tetrahedra connected to metal (M) cations in different coordination
environments MOn (with n = 4, 5 and 6), can generate
pores and channels offering a suitable environment to accommodate various
other cations. Besides their high chemical activity and their thermal
stability (Morozov et al., 2003), such metal-based phosphates
have
some interesting properties leading to applications such as in catalysis
(Cheetham et al., 1999; Viter & Nagornyi, 2009),
ion-exchangers
(Clearfield, 1988; Joschi et al., 2008), gas sorption
(Forster et al., 2003), or batteries (Trad et al.,
2010).

Our interest is particularly focused on hydrothermally synthesized
orthophosphates within the ternary systems MO–M'O–P2O5 with
M and M' = divalent cations. We have recently characterized some
new lead cobalt or manganese phosphates, viz.
Co2Pb(HPO4)(PO4)OH.H2O (Assani et al., 2012a)
and
Pb2Mn3(HPO4)2(PO4)2 (Assani et al., 2012b).
In line with the focus of our research, the present paper describes the
hydrothermal synthesis and the structural characterization of a new
strontium manganese phosphate, Sr2Mn3(HPO4)2(PO4)2, that is
isotypic with its lead analogue, Pb2Mn3(HPO4)2(PO4)2. These two
phosphates are characterized by an Mn:P ratio = 3:4, which is rarely
observed, with the exception of some copper-based orthophosphates,
Pb3Cu3(PO4)4 and Sr3Cu3(PO4)4 (Effenberger, 1999), also
with Cu:P = 3:4.

In the structure of the title compound, one of the two manganese sites (Mn1) is
located on a centre of inversion, while all remaining atoms are in general
positions. A part of the structure, as given in Fig. 1, shows the different
types of polyhedra around the metal positions and the P atoms. The
centrosymmetric Mn1O6 octahedron is linked to two distorted Mn2O6 octahedra
by a common edge, thus forming infinite zigzag chains with composition
[Mn3O14]∞ running parallel to [010] (Fig. 2). Adjacent chains are
linked
to each other through PO4 and PO3OH tetrahedra, via common corners
or edges, leading to the formation of layers parallel to (100). The
cohesion of the crystal structure is ensured on one hand by the presence
of the Sr2+ cations in the interlayer space and on the other hand by strong
O—H···O hydrogen bonds between sheets (Fig. 2 and Table 2).

In the structure of the title compound, the Sr2+ cation is surrounded
by nine O atoms instead of eight as in the case of
Pb2Mn3(HPO4)2(PO4)2. All other bond lengths and angles are similar
in the two structures, with the exception of the Mn2—O bond lengths. In
the title structure, four medium-long bonds in the range 2.1189 (10) to
2.1875 (9) Å and two longer bonds of 2.4079 (12) and 2.4609 (11) Å are observed,
whereas in the lead analogue five medium-long bonds in the range 2.094 (4) to
2.235 (4) Å and one considerably long bond of 2.610 (4) Å is observed.

Transparent crystals of Sr2Mn3(HPO4)2(PO4)2
were isolated from hydrothermal treatment of the reaction mixture of
strontium, manganese, sodium and phosphate precursors in a proportion
corresponding to the molar ratio Sr: Mn: Na: P: = 4: 4.5: 1: 6. The
hydrothermal reaction was conducted in a 23 ml Teflon-lined autoclave filled to
50% with distilled water and under autogenously pressure at 473 K for five
days. After being filtered off, washed with deionized water and air-dried, the
reaction product consisted of colourless crystals with a platy form.

The O-bound H atom was initially located in a difference map and refined with
O—H distance restraint of 0.82 (1) Å. In the last cycle it was refined in
the riding model approximation with Uiso(H) set to 1.5Ueq(O). The
highest peak and the deepest hole in the final Fourier map are at 0.64 Å and
0.55 Å, from Mn2.

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are
estimated using the full covariance matrix. The cell s.u.'s are taken into
account individually in the estimation of s.u.'s in distances, angles and
torsion angles; correlations between s.u.'s in cell parameters are only used
when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.

Refinement. Refinement of F2 against all reflections. The weighted R-factor
wR and goodness of fit S are based on F2, conventional
R-factors R are based on F, with F set to zero for
negative F2. The threshold expression of F2 >
2σ(F2) is used only for calculating R-factors(gt) etc.
and is not relevant to the choice of reflections for refinement.
R-factors based on F2 are statistically about twice as large
as those based on F, and R- factors based on all data will be
even larger.